This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The printed wiring board (PWB) is the foundation for virtually all electronics. The PWB is the platform upon which electronic components such as integrated circuit chips and discrete passive components are mounted. The PWB, sometimes referred to as a printed circuit board (PCB), provides the physical structure for mounting and holding electronic components as well as the electrical interconnection between components. A PWB includes a non-conducting substrate (typically fiberglass, epoxy, and/or polyamide) upon which a conductive pattern or circuitry is formed. Copper is the most prevalent conductor, although nickel, silver, tin, tin-lead, and gold may also be used as etch-resists or top-level metal. There are three types of PWBs: single-sided, double-sided, and multilayer. Single-sided PWBs have a conductive pattern on one side only, double-sided boards have conductive patterns on both sides (top and bottom), and multilayer boards contain two or more double-sided PWBs that are bonded together. The conductive pathways or traces and other features are connected by plated through-holes or vias, which are also used to mount and electrically connect components. PWBs may be rigid, flexible or flexi-rigid.
A variety of processes have been used for forming the conductive pathways on the non-conductive substrate of PWBs. For example, a metal film such as copper can be applied to the non-conductive substrate of the PWB. In a common process, a sheet of the conductive metal is laminated to the non-conductive substrate and a photoresist is then coated on the metal sheet. The resulting PWB is then exposed to a pattern of light employing a light mask to reproduce the metal pathway pattern desired. This exposure is followed by photoresist development and then metal etching in the areas unprotected by the photoresist, in order to produce the desired circuit pattern. In the alternative, an etch resist can be directly printed such as by silk screen on the metal laminate sheet followed by curing and then metal etching. This multi-step process is time-consuming and relatively expensive.
A soldermask, also termed solder resist, is a permanent coating of a resin formulation, typically translucent green in color, which encapsulates and protects all of the surface features of a PWB except the specific areas where it is required to form solder joints. The soldermask is applied to prevent wetting by molten solder and/or liquid solder paste from migrating beyond the desired contact pads during manufacture. The soldermask is spread across the surface of the PWB and so it also provides electrical insulation of the various circuit traces which lie beneath it to prevent them from shorting to one another. The soldermask also prevents the conductors underneath it from shorting with other conductors that may come into contact with the outer surface of the PWB. For the finished PWB the soldermask layer additionally provides long-term protection against oxidation and corrosion of the conductive traces. Historically soldermask has also been used in wave soldering.
Relevant background teachings may be seen in a paper entitled COATES CARBON INKS FOR PCB APPLICATIONS, by R. Smith (July 2001). The color view of that reference shows a translucent green overcoat to the PWB which conventionally represents a soldermask.